"Think of MPV (Multi-Purpose Vehicle) - you would say you own 'AN MPV' not A MPV, I expect?"

Fair point indeed. BUT, MPV is clearly quicker to say than multi-purpose vehicle and therefore MPV is used because it is less of a mouthful. Yet MNer is no quicker to say than Mumsnetter - it is just used as it is quicker to type - so you might as well say Mumsnetter, since that's what is stands for.